EEVEE seem not to benefit from multiple GPUs for rendering
Multi GPU support for EEVEE is a really hard goal to atchieve because of the nature of the technology. It is not being worked on at the moment and it is not planned for any time soon. EEVEE might benefit from SLI or Crossfire setups. It would be logical to wait untill it's realeased to test that though. I believe it is safe to assume that EEVEE will definitely not be able to use multiple GPUs that are not connected with SLI/Crossfire for real time rendering when it is released or any time soon after.